HR: 1340h
AN: T23A-0535    [Abstracts]
TI: Independent Confirmation of a Sharp Lithosphere-Asthenosphere Boundary in Eastern North America Using S-to-P Scattered Waves
AU: * Rychert, C A
EM: Catherine_Rychert@Brown.edu
AF: Brown University, Department of Geological Sciences Box 1846 324 Brook Street, Providence, RI 02906 United States
AU: Rondenay, S
EM: Rondenay@mit.edu
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences 77 Massachusetts Avenue, 54-512 , Cambridge, MA 02139 United States
AU: Fischer, K M
EM: Karen_Fischer@Brown.edu
AF: Brown University, Department of Geological Sciences Box 1846 324 Brook Street, Providence, RI 02906 United States
AB: Scattered waves provide high resolution imaging of crust and upper-mantle discontinuity structure. New results using S-to-P (Sp) scattered waveforms confirm the results from P-to-S (Ps) waveforms, indicating the existence of a very sharp seismic velocity gradient at the lithosphere-asthenosphere boundary in eastern North America. Such sharp velocity gradients provide fundamental constraints on the physical and chemical properties that define the lithosphere-asthenosphere boundary. Using Ps scattered phases we have imaged a lithosphere-asthenosphere boundary that dips gradually northwestward from 90 to 110 km in eastern North America at stations HRV, LMN, BINY, LBNH, SSPA, and PAL. The modeling of Ps waveforms observed at stations HRV and LMN indicates that the velocity gradient associated with the boundary is strong and sharp: a 3-11% drop in shear wave velocity that occurs over 11 km or less. Sp conversions provide not only an independent confirmation of the existence of the discontinuity, but also independent constraints on the seismic velocity gradient at the boundary. Although S-waves are generally noisier than P-waves, reverberations are absent in Sp imaging because direct conversions arrive before the incident S-wave. Sp imaging at stations HRV and LMN reveals a discontinuity at a depth consistent with that observed by Ps imaging. Modeling Sp conversions from the lithosphere-asthenosphere boundary reveals a velocity gradient which is generally consistent with the Ps results both at HRV (3.1-5.7% drop in shear wave velocity in less than 5 km depth at ~97 km depth) and LMN (6.8-11% drop in shear wave velocity in 11 km or less at ~90 km depth). Experimental studies suggest that temperature increases of at least 220 °C and 120 °C are required to explain the velocity contrasts at LMN and HRV, respectively. However, numerical models of mantle flow in which viscosity depends only on temperature and pressure indicate that the thermal gradient at the base of the lithosphere is typically less than 5 °C/km, and definitely less than 10 °C/km, too gradual to explain the observed velocity contrast. Therefore, the lithosphere-asthenosphere boundary in this region is not defined by temperature alone, and requires another mechanism such as dehydration, depletion, or melt. Although, taken separately, depletion or dehydration may be too small to explain the upper limits of the observed contrast, the combination of a depleted lithosphere with a hydrated asthenosphere can explain the velocity contrast at HRV and reach the LMN range with a modest contribution from temperature. Alternatively, a small amount of partial melt in the asthenosphere can easily explain the observed strong, sharp velocity contrast. One mechanism for generating melt beneath eastern North America is decompression of mildly hydrated asthenospheric material as it flows upward along the contours of the more rigid, shallowing lithosphere.
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8110 Continental tectonics: general (0905)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Tectonophysics [T]
MN: Fall Meeting 2005